The present invention relates to a photoconductor for electrophotography (hereinafter referred to as an "electrophotographic photoconductor" or simply as a "photoconductor"). Specifically, the present invention relates to an electrophotographic photoconductor that includes a photosensitive layer of organic materials on an electrically conductive substrate which is used in printers and copying machines that employ electrophotographic techniques.
Conventional photoconductors for printers, facsimiles and copying machines using electrophotographic techniques include an inorganic photoconductive material such as selenium or alloys of selenium. Other inorganic photoconductive materials such as zinc oxide and cadmium sulfide dispersed in a resin binder have also been used. Recently, research and development has been performed to investigate the use of organic photoconductive materials in electrophotographic photoconductors. Some organic electrophotographic photoconductors exhibiting improved sensitivity and durability have been developed.
Photoconductors must retain surface charges in the dark, generate charges in response to received light and transport charges in response to the received light. The so-called single-layer-type photoconductor includes a layer that exhibits all the aforementioned functions. The so-called laminate-type photoconductor includes a laminate consisting of a layer that contributes to charge generation and a layer that contributes to surface charge retention in the dark and charge transport under light exposure.
Electrophotography using the aforementioned photoconductors can form images using, for example, the Carlson process. The Carlson process includes the steps of electrifying the photoconductor by corona discharge in the dark, forming electrostatic latent images of original letters and pictures by light irradiation onto the electrified photoconductor, developing the latent images with toner, and fixing (copying) the developed toner images on a carrier such as paper. After the toner images are copied, the charges on the photoconductor are removed, the residual toner is removed, the optical charge is removed and the photoconductor is prepared for the next image formation.
The organic photoconductors developed so far are superior to the inorganic photoconductors in flexibility, ease of film formation, low manufacturing costs and safety. Further improvements of the sensitivity and durability of the organic photoconductors have been studied using a variety of organic materials.
Most of the organic photoconductors are of a laminate-type which distributes the foregoing basic functions among a charge generation layer and a charge transport layer. Usually, the laminate-type photoconductor includes an electrically conductive substrate; a charge generation layer on the substrate, containing a charge generation agent such as pigment or dye; and a charge transport layer, containing a charge transport agent such as hydrazone and triphenylamine on the charge generation layer. Due to the electron donating nature of the charge transport agent, the laminate-type photoconductor is usually a hole-transport-type, which exhibits sensitivity when its surface is negatively electrified. The corona discharge for negative electrification is more unstable than that for positive electrification and generates ozone and nitrogen oxide. The ozone and nitrogen oxide that are generated, absorb to the photoconductor surface, resulting in deterioration of the photoconductor physically and chemically. The ozone and nitrogen oxide are also hazardous to the environment. The photoconductor of a positive-electrification-type is superior to the negative-electrification-type of photoconductor, since the positive-electrification-type photoconductor can be used in a greater variety of working conditions. The fields to which the positive-electrification-type photoconductor is applicable are wider.
Due to the limitations of the negative-electrification-type photoconductors, research has been done to develop photoconductors for positive electrification. For example, single-layer-type photoconductors has been proposed which includes a photosensitive layer where a charge generation agent and a charge transport agent are dispersed in a resin binder. Some of these are used practically. However, the single-layer-type photoconductor is not sensitive enough to be applicable to high-speed machines. In addition, the single-layer-type photoconductor must have sufficient stability during repeated use.
To improve sensitivity, a positive-electrification-type photoconductor has been proposed which has a function-separation-type laminate with a charge generation layer laminated on a charge transport layer. However, corona discharge, light irradiation and mechanical wear interfere with stability under repeated use, since this type of photoconductor has a charge generation layer in its surface. Placing a protection layer on the charge generation layer has been proposed to obviate the aforementioned problems. Although the protection layer improves mechanical wear resistance, it interferes with the ability to improve the electrical properties of the photoconductor, including sensitivity.
A positive-electrification-type photoconductor that includes a function-separation-type laminate of a charge transport layer containing an electron transport agent, on a charge generation layer, has also been proposed. The electron transport agent such as 2,4,7-trinitro-9-fluorenone is known to those skilled in the art. However, this electron transport agent is carcinogenic and hazardous to human health. Although other electron transport agents such as cyanine compounds and quinone compounds have been proposed (cf. Japanese Unexamined Laid Open Patent Applications No. S50-131941, No. H06-59483, No. H06-123986 and No. H09-190003), a compound exhibiting sufficient electron transport capability has not yet been obtained.